1. Field of the Invention
The present invention relates to a ceramic multi-layer wiring board, and a process for the preparation thereof. More particularly, the present invention relates to an improved ceramic multi-layer wiring board, and a process for the preparation thereof, whereby a stable electroconductivity is maintained.
2. Description of the Related Art
The conventional ceramic multi-layer wiring board is roughly divided into a printed multi-layer board and a laminated multi-layer board, and the conventional ceramic multi-layer wiring board is prepared according the process described below.
This preparation process will be described with reference to the printed multi-layer board as an example. First, an insulating paste for forming an insulating layer and a conductor paste for forming a refractory metal conductor layer are alternately printed on a ceramic green sheet to form a plurality of insulating layers and a plurality of conductor layers, and these layers are simultaneously fired in a reducing atmosphere to obtain a multi-layer wiring board having an exposed conductor layer. Thereafter, a conductor paste for mounting parts and forming thick film elements is printed on the exposed conductor layer of the multi-layer wiring board, and the conductor paste is fired in an oxidizing atmosphere to obtain a ceramic multi-layer wiring board.
More specifically, we shown in FIG. 4 illustrating an example of the multi-layer wiring board having an exposed conductor layer, which is obtained by forming a multi-layer wiring portion by the printing method and carrying out the simultaneous firing in a reducing atmosphere, a wiring pattern Ml is formed on a green sheet or a green sheet 1 having a multi-layer wiring installed therein by using a conductor paste for forming a conductor layer, and then an insulating paste Z1 is printed thereon to form an insulating layer. Thereafter, a via hole-embedding pattern M2 is printed by a conductor paste for forming via holes, composed of a refractory metal, an insulating paste Z2, a conductor paste M3, and an insulating paste Z3 are printed in sequence, and then firing is conducted in a reducing atmosphere to prevent oxidation of the conductor paste. Then, for example, a nickel plating 2 and an oxidation-resistant barrier comprising an Au/Ag alloy layer 3 are formed on the surface of the conductor layer M3, except for the surface of the portion of the insulating layer Z3, to prevent oxidation of the inner conductor, and then, a conductor paste for mounting parts and forming thick film elements is printed. The conductor paste is fired in an oxidizing atmosphere to obtain an intended ceramic multi-layer wiring board.
The insulating paste and the conductor paste are alternately printed because for example, if a plurality of layers are continuously formed by printing the insulating paste, the level difference become accordingly larger, and when the conductor paste is then printed, an unsatisfactory insulation or a formation of printing faults occurs.
In the above-mentioned conventional ceramic multi-layer wiring board, as shown in an enlarged sectional partial view of FIG. 5, among the simultaneously fired insulating layers of the multi-layer wiring board, the end portion T of the outermost insulting layer Z3 becomes thin because of a sagging and bleeding of the printed insulating paste. The thickness t.sub.0 of the insulating layer Z3 is 10 to 20 .mu.m, but sometimes the thickness t.sub.1 of the end portion T is about 1 to about 2 .mu.m. In this state, since pinholes are formed or the number of ceramic particles is reduced in this end portion T, the possibility of a formation of defects through the spaces among the ceramic particles in this insulating layer is very high. Moreover, the inventors, found that, since the oxidation-resistant barrier layer composed of the nickel plating 2 and the Au/Ag alloy 3 is not formed on this portion, the problem of oxidation of the end portion of the inner conductor layer M3 arises during the firing conducted in a high-temperature oxidizing atmosphere. This oxidation cause various problems to arise, such as a disconnection of the conductor pattern and lowering of the electroconductivity, and further, the problem of a reduction of the manufacturing yield arises.